I would leave most of the plastic case exposed. I would get some outdoor silicone
caulking and encapsulate the bottom of the case including the package leads and
the solder connections.
How did you solder the wire leads to the package leads? For an application that will
be exposed to the elements it is important to have a mechanically stable connection
prior to soldering. I would either tightly wrap the lead and wire or loop the wire
around a loop you bend in the lead. You could also mount the sensor on a small piece of vectorboard.
(* jcl *)